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Abstract

Historically, vertical fixation disparity curves have been described as straight lines. This suggests minimal prism adaptation to vertical prism stress. There have been reports in the literature, and it has been our clinical experience, that while many vertical fixation disparity curves are straight lines, many others are sigmoidal. This study evaluated the slopes of thirty vertical fixation disparity curves from non-asthenopic subjects to see if the magnitude of the change in slope is striking enough to identify the curves as something other than a straight line. The results indicate that the majority of the graphs are straight lines, as most literature suggests, with a total of twenty straight line curves and ten sigmoid shaped. However, two sigmoidal curves show a large magnitude of change across the slope suggesting vertical prism adaptation is indeed possible.